Hristiyan A. Aleksandrov

How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H(sub) , may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H(ad) , depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H(sub) , especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.

Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture

Palladium is crucial for industry-related applications such as heterogeneous catalysis, energy production, and hydrogen technologies. In many processes, atomic H and C species are proposed to be present in the surface/near-surface area of Pd, thus noticeably affecting its chemical activity. This study provides a detail and unified view on the interactions of the H and C species with Pd nanoparticles (NPs), which is indispensable for insight into their catalytic properties. Density functional calculations of the interplay of C and H atoms at various concentrations and sites on suitable Pd NPs have been performed, accompanied by catalysis-relevant experiments on oxide-supported bare and C-modified Pd NPs. It is shown that on a Pd(79) NP a subsurface C atom destabilizes nearby atoms H at low coverage. Our experiments confirm that H atoms bind more weakly on C-containing Pd NPs than on C-free NPs. Various factors related to the presence of both H and C atoms on a Pd(79) surface, which may influence the penetration of H atoms from the surface into the subsurface area, have been investigated. Carbon atoms facilitate the subsurface penetration of atomic H both thermodynamically and kinetically when the surface is densely covered by H atoms. Moreover, subsurface H atoms are also energetically favored, even in the absence of C atoms, when several facets of the NP are covered by H atoms.

Theoretical investigation of Zn-containing species in pores of ZSM-5 zeolites

We report computational studies of various cationic Zn-containing species in pores of ZSM-5 zeolite. The calculations were performed at the gradient-corrected density functional level employing both isolated and embedded quantum clusters for modeling the zeolite framework. We studied zinc species in two models M5 and M7, comprising one and two coupled 5-membered rings with two Al centers. Our results suggest that ZnOH+ species can be transformed into Zh(H2O)2+ complexes when a bridging OH group is available nearby in the zeolite framework. The desorption energy of H2O molecules from the later complexes is rather high, 184 (M5) and 130 (M7) kJ/mol. Zn-containing species at the smaller zeolite ring are further stabilized by additional OH− or H2O ligands. We found the formation of ZnOZn2+ species energetically unfavorable.

One-pot synthesis of silanol-free nanosized MFI zeolite

The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Here, we report and characterize, using a combination of experimental and theoretical techniques, the one-pot synthesis of silanol-free nanosized MFI-type zeolites by introducing atomically dispersed tungsten; this prevents silanol group occurrence by forming flexible W-O-Si bridges. These W-O-Si bonds are more stable than Si-O-Si in the all-silica MFI zeolite. Tungsten incorporation in nanosized MFI crystals also modifies other properties such as structural features, hydrophobicity and Lewis acidity. The effect of these is illustrated on the catalytic epoxidation of styrene and separation of CO2 and NO2. Silanol-free nanosized W-MFI zeolites open new perspectives for catalytic and separation applications.

Elucidation of the higher coking resistance of small versus large nickel nanoparticles in methane dry reforming via computational modeling

Dry reforming of methane (DRM) is a promising utilization process of greenhouse gases, namely CO2 and CH4. Nickel-based catalysts are the most popular ones for DRM, because they are inexpensive and relatively active but deactivate rapidly mainly due to carbon formation. Carbon gasification by either partial or complete oxidation is considered to be the main route for carbon-free catalysis. To clarify the competition between carbon deposition and carbon gasification in this computational study, we modeled the formation of C–C and C–O bonds on large and small Ni-nanoparticles (∼1 nm). We found that C prefers to penetrate into the subsurface, whereas O prefers to adsorb on the surface of both large and small nickel particles. The formation of CO at low concentrations is significantly more exothermic than C2 formation but the C2 moiety is formed faster than CO. At low carbon concentrations, the formation of the C–C bond is not favorable with respect to the two C species located in the subsurface region. However, at high C concentrations, on the large metal particles, multicarbon Cn species are formed; those species are potential precursors of carbon deposits such as graphene or coke. On the other hand, the flexibility of the small nickel nanoparticles allows separation of monoatomic C to remain stable as subsurface species. Thus, Cn species, considered to be precursors of carbon deposits leading to catalyst deactivation, are found to preferably form on large rather than on small Ni nanoparticles.

Relative stability and reducibility of CeO2 and Rh/CeO2 species on the surface and in the cavities of γ-Al2O3: a periodic DFT study

We report the structure and stability of ceria units deposited on the surface of γ-Al2O3 or incorporated in its cavities, as determined by periodic density functional calculations. Ceria species are modeled as CeO2 or Ce2O4 moieties or as a small nanoparticle, Ce13O26, on the (100) and (001) surfaces of a γ-Al2O3 slab. Among the studied structures the incorporation of Ce(4+) ions in cavities of γ-Al2O3 is favored with respect to the ions on the surface only in subsurface cavities of the (100) surface. The calculations also suggested that formation of a surface layer of ceria on the (100) alumina surface is preferable compared to three-dimensional moieties. The deposition of a small ceria nanoparticle on (100) and (001) surfaces of γ-Al2O3 reduces the energy for oxygen vacancy formation to an essentially spontaneous process on the (100) surface, which may be the reason for the experimentally detected large fraction of Ce(3+) ions in the CeO2/γ-Al2O3 systems. The deposition of a single rhodium atom or RhO unit in some of the structures with a CeO2 unit and Ce13O26 showed that spontaneous electron transfer from rhodium to cerium ion occurs, which results in reduction of Ce(4+) to Ce(3+) and the oxidation of rhodium. Only in the presence of deposited rhodium atoms, the incorporated cerium ions can be reduced to Ce(3+).

Inhibition of Palm Oil Oxidation by Zeolite Nanocrystals

The efficiency of zeolite X nanocrystals (FAU-type framework structure) containing different extra-framework cations (Li(+), Na(+), K(+), and Ca(2+)) in slowing the thermal oxidation of palm oil is reported. The oxidation study of palm oil is conducted in the presence of zeolite nanocrystals (0.5 wt %) at 150 °C. Several characterization techniques such as visual analysis, colorimetry, rheometry, total acid number (TAN), FT-IR spectroscopy, (1)H NMR spectroscopy, and Karl Fischer analyses are applied to follow the oxidative evolution of the oil. It was found that zeolite nanocrystals decelerate the oxidation of palm oil through stabilization of hydroperoxides, which are the primary oxidation product, and concurrently via adsorption of the secondary oxidation products (alcohols, aldehydes, ketones, carboxylic acids, and esters). In addition to the experimental results, periodic density functional theory (DFT) calculations are performed to elucidate further the oxidation process of the palm oil in the presence of zeolite nanocrystals. The DFT calculations show that the metal complexes formed with peroxides are more stable than the complexes with alkenes with the same ions. The peroxides captured in the zeolite X nanocrystals consequently decelerate further oxidation toward formation of acids. Unlike the monovalent alkali metal cations in the zeolite X nanocrystals (K(+), Na(+), and Li(+)), Ca(2+) reduced the acidity of the oil by neutralizing the acidic carboxylate compounds to COO(-)(Ca(2+))1/2 species.

Computational evaluation of the capability of transition metal exchanged zeolites for complete purification of hydrogen for fuel cell applications: the cheapest performs the best

The study reports an estimation of the capability of zeolites exchanged with different monovalent transition metal cations to be used for the deep purification of hydrogen for PEMFC applications from CO, ammonia and hydrogen sulfide impurities. The estimation is based on thermodynamic data derived by computational modeling using the periodic DFT approach, and allows determination of the minimal impurity gas concentration in the H2 feed that could be achieved with the specific adsorbent. The results suggest that zeolites exchanged with Co+, Ni+, Cu+, Rh+ or Ir+ cations can purify hydrogen under standard conditions to CO concentrations of 10−12–10−16, depending on the metal. A theoretically recommended material for hydrogen purification is Cu exchanged zeolite, since it is able to reduce CO concentrations down to 10−11 and has weaker binding of CO compared to the other modeled cations, which facilitates the regeneration of the adsorbent. In addition, this zeolite can capture NH3 and H2S impurities and reduce their concentration in the H2 feed to 10−10, while the other modeled cation exchanged zeolites show a higher permeability of these impurities and are less appropriate. Thus, the modeled adsorbents, in particular Cu exchanged zeolite, are good candidates for H2purification due to their low cost and predicted high efficiency, and could be considered as an appropriate alternative to the other currently applied approaches.

Decomposition behavior of platinum clusters supported on ceria and γ-alumina in the presence of carbon monoxide

Since the catalytic behavior of supported platinum species strongly depends on their nuclearity under the reaction conditions, in this study we addressed the stability of platinum species in the presence of CO. We applied density functional modeling to clarify the effect of the CO coverage on the structure and stability of small platinum clusters deposited on CeO2(111) and γ-Al2O3(001) surfaces and on a ceria nanoparticle. The stability was evaluated with respect to decomposition of the clusters to Pt, Pt0(CO) and/or Pt2+(CO)2 species. The results suggest that decomposition of pristine platinum clusters to Pt0 atoms is endothermic on all considered supports. On the other hand, the formation of Pt0(CO) monocarbonyls is essentially energy-neutral when the platinum cluster deposited on the ceria nanoparticle or ceria (111) surface is fully covered by CO molecules. Formation of Pt2+(CO)2 species can occur only when the support is a small ceria nanoparticle which ensures low-coordinated O centers and easily reducible Ce4+ ions. On the γ-Al2O3(001) surface, the platinum cluster remains stable with respect to decomposition even when it is fully covered by CO but the interaction of the cluster with the alumina surface is strongly weakened. Thus, based on the results, one may suppose that under CO pressure platinum clusters behave differently depending on the support – decomposition to neutral monocarbonyls on the ceria surface and to cationic complexes on the ceria nanoparticle, while on alumina the carbonylated cluster remains intact but is almost detached from the surface.

From Static to Reacting Systems on Transition-Metal Surfaces

Comprehensive density functional studies provide important insights into details of reaction mechanisms on surfaces that would be very hard to obtain experimentally. This chapter presents a critical comparison of experimental and theoretical achievements in mechanistic descriptions of the following surface reactions (1) (de-)hydrogenation of unsaturated hydrocarbons on Pd, Pt, and Rh; (2) methanol decomposition and steam reforming on Cu, Pd, and PdZn; (3) selective hydrogenation of unsaturated aldehydes on pure and alloyed Pt as well as Ag and Au surfaces; and (4) CO oxidation on nanoporous gold.